Maintain a stack of values of the complete subexpressions encountered so far, with the newest value at its head:
evalRP :: RPolish -> Int
evalRP = finish . foldl push []
  where
    push :: [Int] -> Elem -> [Int]
    push stack (Number n) = n : stack
    push (right:left:stack) (Operator op) =
      applyOp op left right : stack
    push _ (Operator _) = error "missing operand"

    finish :: [Int] -> Int
    finish [value] = value
    finish _       = error "malformed expression"
The left fold processes tokens in their order. A number pushes its own value; an operator replaces the two latest subexpression values by their combination. The top value is the right operand, so preserving the order in applyOp op left right is necessary for subtraction and division. Induction on the consumed token prefix proves this stack machine invariant. A complete valid expression finishes with exactly one value.

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